Build Cython in parallel on "make local".

Makefile

@@ -3,6 +3,7 @@ PYTHON?=python
 TESTOPTS?=
 REPO = git://github.com/cython/cython.git
 VERSION?=$(shell sed -ne 's|^__version__\s*=\s*"\([^"]*\)".*|\1|p' Cython/Shadow.py)
+PARALLEL?=$(shell ${PYTHON} -c 'import sys; print("-j5" if sys.version_info >= (3,5) else "")' || true)
 
 MANYLINUX_IMAGE_X86_64=quay.io/pypa/manylinux2010_x86_64
 MANYLINUX_IMAGE_686=quay.io/pypa/manylinux2010_i686
@@ -10,10 +11,10 @@ MANYLINUX_IMAGE_686=quay.io/pypa/manylinux2010_i686
 all:    local
 
 local:
-	${PYTHON} setup.py build_ext --inplace
+	${PYTHON} setup.py build_ext --inplace $(PARALLEL)
 
 plocal:
-	${PYTHON} setup.py build_ext --inplace --cython-profile
+	${PYTHON} setup.py build_ext --inplace --cython-profile $(PARALLEL)
 
 sdist: dist/$(PACKAGENAME)-$(VERSION).tar.gz